The present invention relates in general to laser technology. More particularly, the present invention provides an apparatus and method for determining the position, speed and direction of motion of reflective targets, as well as for detecting changes in refractive indices of light transmissive gases. According to the present invention, an improved optical disc reader is provided capable of reading at relatively high speeds compared to prior art optical disc readers and/or capable of operation with extremely small pit depths. The present invention also facilitates improved transmission and reception through fiberoptic cable networks.
The prior art includes the Bearden et al U.S. Pat. No. 5,029,023 dated Jul. 2, 1991 which teaches a laser motion detector utilizing a single laser cavity and laser feedback interferometry to measure displacements in a target surface. In contrast to the teachings of the Bearden '023 patent, the present invention utilizes in one embodiment a pair of matched, monolithic diode lasers, wherein the first laser transmits an output beam and the second laser receives the reflected beam from a target. The present invention differs from Bearden '023 in several significant respects. First, the present invention utilizes in several embodiments matched diode lasers, using two or more laser cavities, wherein the output beams are easily modulated electronically. The Bearden '023 patent teaches the use of a single laser cavity which must be modulated mechanically or electro-optically and which cannot be effectively modulated electronically. Secondly, the present invention incorporates dual frequency injection to detect a target whereas Bearden '023 teaches a single frequency injection to stabilize the device (see column 7, line 53 thru column 8, line 19). Thirdly, the present invention in several embodiments includes the second harmonic as part of a compound modulated operating signal to detect the target, whereas Bearden '023 does not use harmonics in any fashion to detect the target.
The prior art also includes the Bearden et al U.S. Pat. No. 5,235,587 dated Aug. 10, 1993 which discloses a method and apparatus for storing and retrieving data from an optical disc using multiple pit depths. The Bearden '587 patent differs from the present invention in that it utilizes a single laser cavity which requires optical modulation of the beam to maintain stability. The use of a single laser cavity as taught in Bearden '587 has certain inherent problems in operating at relatively high frequencies. The primary problem is that the feedback light momentarily decreases the output power of the single laser cavity, requiring that before the next bit is retrieved, the laser must be allowed to return to its original operating power. At higher frequencies, the laser does not have adequate time to return to its original output power and would have inherent difficulty in detecting a string of zeroes, for example.
The prior art also includes Bearden et al U.S. Pat. No. 5,260,562 dated Nov. 9, 1993 which teaches a high resolution light microscope. This patent includes the same disadvantages of the other Bearden prior art summarized above.